1. Field of the Invention
Embodiments of the present invention relate generally to gas sensors, and more specifically to highly sensitive gas sensors utilizing carbon nanotubes and integrated antennas for wireless applications.
2. Background of Related Art
Carbon nanotubes (CNTs) are long thin cylinders of carbon with a typical diameter ranging from approximately 1 nm to 100 nm. Carbon nanotubes have a number of advantageous properties including, for example, their size, shape, and physical properties. Due to their large tubular surface area, with a hollow inner core and a thin cylindrical wall, CNTs exhibit moderate to high surface adsorption. This adsorption, in turn, can produce appreciable changes in electrical conductivity of the CNTs at room temperature in the presence of various substances.
Conventional CNT-based gas sensors, which generally rely on detecting either a change in the resonant frequency or change in the amplitude upon exposure to the analyte of interest, provide insufficient sensitivity for many applications. Previous designs have included, for example, a 3.9 GHz patch resonator coated entirely with a mixture of single-walled nanotubes (SWNT) in powder form for ammonia detection.1 When the CNT coating is exposed to ammonia, it changes the effective permittivity and shifts the resonant frequency of the resonator. Unfortunately, minimal frequency shifts have been detected (approximately 5-MHz), even in the presence of high ammonia concentration (e.g., up to 1000 ppm). Similar sensors have also reported a shift of approximately 7 MHz when the resonator was completely immersed in methanol.2 1 S. Chopra, A. Pham, J. Gaillard, A. Parker, and A. M. Rao, “Carbon-nanotube-based resonant-circuit sensor for ammonia,” Appl. Phys. Lett., vol. 80, no. 24, pp. 4632-4634, June 2002.2 Y. Zhou, Y. Bayram, F. Du, L. Dai, and J. L. Volakis, “Polymer-carbon nanotube sheets for conformal load bearing antennas,” IEEE Trans. Antennas Propag., vol. 58, no. 7, pp. 2169-2175, July 2010.
While these shifts are detectable in a laboratory setting, in a remote sensing mode, such small detection shifts can lead to false alarms. Indeed, variances in the resonator fabrication due to manufacturing tolerances may cause changes of the same magnitude. Furthermore, the large losses in the resonator, caused by CNT loading, limit sensor sensitivity and preclude effective integration of wireless transmission devices such as antennas. As a result, these configurations are not suitable for low-cost and/or wireless applications for standoff sensing, for example.
Other sensors have been manufactured comprising a composite of multi-walled nanotubes (MWNTs) and SiO2 (which simply acts to bind the MWNTs) placed over a planar LC-resonator printed on silicon. This configuration can be fabricated by photolithography on a printed circuit board substrate.3 The sensor detects the change in the effective dielectric constant of the circuit caused by surface interaction with the gas. Again, due to very small variation in the dielectric constant upon exposure to gas, however, this sensor also suffers from low sensitivity, thus limiting its practical uses. 3 K. G. Ong, K. Zheng, and C. A. Grimes, “A wireless passive carbon nanotube-based gas sensor,” IEEE Sensors J., vol. 2, no. 2, pp. 82-88, April 2002.
Still other types of CNT-based gas sensors rely on the change in amplitude of the reflected signal (i.e., the return loss) or the transmitted signal for detection. Unfortunately, this method is susceptible to interference and noise, which can also lead to false readings. Conversely, relying on the shift of the resonant frequency—assuming the shift in frequency is sufficiently large to enable accurate detection—is effective for remote sensing because the frequency shift is relatively insensitive to detrimental influences such as noise and interference.
What is needed, therefore, is a gas sensor utilizing the advantageous properties of CNTs. The sensor should take advantage of the relatively stable shift in resonant frequency, yet improve the sensitivity thereof. The sensor should use conventional manufacturing techniques to provide improved, low-cost gas detectors. The sensor should incorporate a low-cost, high sensitivity antenna to enable accurate detection and wireless transmission over standoff distances of several meters to several kilometers. It is to such a system that embodiments of the present invention are primarily directed.